A Qantas Airways plane made an emergency landing in Manila on Friday after part of its undercarriage blew off, triggering a loss in cabin pressure during a flight from Hong Kong to Melbourne.

Passengers reported hearing a loud bang before the aircraft rapidly lost altitude and said the Boeing 747-400 had a hole the size of a mini-van on the right of its undercarriage when it landed in the Philippine capital.

"It was an amazing experience, but not a good one," Glenyce Johnson, 47, told Reuters.

The Australian Transport Safety Bureau said QF30 made an emergency descent from 29,000 feet to 10,000 feet. All 346 passengers and 19 crew disembarked safely. Experts from Australia were travelling to the Philippines to inspect the plane.

Johnson, who was travelling home to Australia, saw items flying out of the plane. Oxygen masks were released.

"I have to compliment the pilot for doing a good job, for safely landing the plane," she said.

John Vanharen, a sheep farmer from Tasmania, was unperturbed by the incident.

"That's no big deal," he said.

QF30 left Hong Kong at 9 a.m. (2 a.m. British time) and was due to land in Melbourne at 9:45 p.m. (12:45 p.m British time). The flight originated in London.